Tho my work is generally done in stone and glass, or fine art stuff, but sometimes as a favor I do work up designs for tats and bikes... Always... I talk to the client and get a sense of who they are and what matters/resonates with them... There are no rules, it all about what feels right for you and what reflects who you are. It could be something that feels right... other times it comes from those you care about... or an animal/design/line/combo that you've seen that you respond to. Sometimes complicated, and other times as simple as shadow and light. Suggest you visit some art sites and identify things that you really like... then find an artist (and I include air-brush folks as artists!!), talk to them, show them the stuff you've found that 'feels' right for you and get them to work up a sketch... good artists will work with you back and forth til you're happy with the basics... then get them to do a sketch of the design for the bike... with the Adobe programs, it's not all that much extra work. Understand that the sketch will NOT reflect the depth and complexity of the finished work... that will just be a whole lot better than the sketch!! In my opinion, Bad Ass is great for celtic inspired work, Goodeve studios in Canada is outstanding in creative, Pam Shanteau very solid... and Fowers (a Forum member) has a great fine art hand... and there are a couple of other guys on the Forum who are equally good with geometric/line/light/shadow defined work... The most important thing is to get a sense of what feels right to you... then find the artist who works best with that theme... and who will work with you til you're happy!! IMO your bike is a mirror of who you are, and the paint needs to reflect that! Good luck in your journey... based on what I've seen you do with your ride in the past, go with what you feel, and you'll do just fine!
